The goals of the investigation proposed here are to examine the molecular interactions between calpain 3 and titin, and to test whether perturbations in those interactions create defects in normal myogenesis. The finding that null mutations in calpain 3 result in debilitating limb girdle muscular dystrophy type 2A, indicates that calpain 3 is important for normal muscle homeostasis. However, the physiological relationship between calpain 3 deficiency and muscular dystrophy is unknown, largely because of lack of information concerning the normal function and interactions of calpain 3. It is proposed here that calpain 3 exists with titin in a macromolecular, functional complex in muscle, and that perturbations of the complex can result in myopathies. Several observations support the contention that defects in titin or its binding partners can cause myopathy. If calpain 3 were also proven to be a titin-binding protein in muscle, that would further advance both our understanding of the basic biology of calpain 3, but also our knowledge of the relationship between defects in the titin macromolecular complex and muscle disease. However, current knowledge of the relationship between titin and calpain 3 is scant, because of the difficulty of isolating the unstable calpain 3 protein. Although it has been demonstrated that calpain binds titin in yeast two-hybrid assays, whether this interaction actually occurs in muscle and whether it is physiologically important remain unknown. The investigation proposed here is designed to examine the relationship between calpain 3 and titin in muscle, and to test the hypothesis that thin interactions with calpain 3 are important for normal myogenesis and muscle structure. The specific aims are: (Aim 1) To test the hypothesis that changes in calpain 3 expression or structure produce alterations in the myogenic phenotype of transgenic mice. (Aim 2) To test the hypothesis that calpain 3 interacts with titin in muscle, and to determine the functionally important domains for that binding. (Aim 3) To test whether C3 interactions with titin are required for normal myogenesis.